ICON, INDEX and SYMBOL (Long Version)

People who study signs and communication differentiate three kinds of signs: an ICON
from an INDEX from a SYMBOL. This distinction is very
important and derives from philosopher C. S. Peirce in the late 19th
century. This page is an attempt to sharpen the difference between
these three which are described in your text (CELL, pp. 1-4).
The critical issue is to appreciate what a symbol is. This is
the key to understanding language and how it differs from any animal
communication systems.

First, we must note that a sign is a stimulus
pattern that has a meaning. The difference between
the various kinds of sign has to do with how the meaning happens
to be attached to (or associated with) the pattern.

ICON

The icon is the simplest since it is a pattern that physically
resembles what it `stands for'.

A picture of your face is an icon of you.

The little square with a picture of a printer on your
computer screen is an icon for the print function. (Whereas
a little box that has the word `PRINT' is not an icon
since it has no physical resemblance to printing or the printer.)

The picture of a smoking cigarette with a diagonal bar
across the picture is an icon that directly represents `Smoking?
Don't do it' (at least it does with appropriate cultural
experience).

Your cat is preparing to jump up on your lap, so you put out the palm
of your hand over the cat to prevent him from jumping. The first time,
you may physically impede his jump (This is not a sign at all), but
after a couple times, just putting your palm out briefly
becomes an iconic sign for `You aren't welcome on my lap right
now.' The gesture is an icon because it physically resembles an
act of preventing him from jumping, even though it would not prevent
him if he really wanted to do it.

Words can be partly iconic too. Bow-wow, splash and hiccup
resemble the sounds they represent- at least a little. And the bird
called the whippoorwill produces a call resembling this
English phrase, so whippoorwill is an iconic word. (These are also
called onomotopoetic words.)

Also words can be pronounced in an iconic way: His nose grew
wa-a-a-ay out to here. Julia Childes grabbed that carrot and went CHOP
CHOP CHOP CHOP. Aw, poor widdow ba-by!

Of course sometimes there could be a dispute about what `physical
resemblance' means and how similar it must be. And just because we
humans can recognize a little picture of something doesn't mean that
any other animal could. (Do you think your cat could recognize a
picture of a can of catfood and interpret it as ``Time to eat''? Not
likely.) So physical resemblance is by no means a simple concept. But
how and why an image or a sound has some particular semantic content
for us humans is fairly easy to understand.

INDEX

An `index' is defined by some sensory feature, A, (something
directly visible, audible, smellable, etc) that correlates with
and thus implies or `points to'B, something
of interest to an animal. All animals exploit various kinds of
indexical signs in dealing with the world. The more intelligent animals
are good at learning and exploiting more sophisticated indices (thus a
cat will use and learn many more indexical signs than a frog, a fish or
an ant -- which tend to be restricted to ones acquired innately).

Thus,

dark clouds in the west are an index of impending rain (at
least in Indiana),

for a fish in the sea, the direction of greater light is the
direction of warmer water,

a limping gait is a sign that an animal is physically
impaired,

a scowling facial expression is an index of the person's displeasure
or concern (to a human),

sensing a pheremone in the air is an indexical sign (for
some insects) that a sexually receptive member of its own species
is located upwind,

a particular alarm call in certain monkeys is a sign that
the animal has either directly sensed (eg, seen, smelled, heard) a
particular type of predator OR has heard another monkey give
this predator alarm call.

a particular pronunciation of a word is a index that someone
comes from a particular geographic place or social group.

Note that all of these above depend on a certain statistical
regularity of part A (the signal pattern) with part B (the
behaviorally relevant state). The exploitation of this regularity
requires first, detecting property A (which is not necessarily
simple) and either learning (or innately knowing) its correlation
with the B. In that case the animal will use A as an index
for B.

Note that for humans, some indices can be artificial and manmade
(rather than environmentally natural or innate to particular species):

a beep from your oven can signal that the cookies are
ready to be removed,

a red stoplight is a sign that you should stop your car
if you don't want to risk an accident,

in an animal behavior experiment, a flashing light could be
a sign that food will be available in a certain place or
that a shock will soon follow.

a person can wave their hand as a sign of recognition and
greeting (though this may be partly iconic too).

Notice that the correlation need not be perfect. It isn't always
warmer closer to the sea surface, dark clouds in the west don't always
mean the rain is coming this way, and even a stoplight can be broken
sometimes. This doesn't detract from the usefulness of these signs as a
way for an animal to guide its life in a confusing and only partly
predictable world.

Words are said to be indexical when they directly
point to their meaning - without depending on any relationship to
other words. Thus, words like here, there, I, me, you, this,
etc. For all of these there is an implied pointing gesture. (Remember
in Latin, index really meant the index finger.)

SYMBOL

Words as Symbols.

Now, what about a noun word in a human language? Let's say English
`KITTY'? Isn't this just a kind of arbitrary index? Isn't KITTY
just an index for the presence of a cat (just for English
speakers of course)? In support, one might note that a small child and
its mother would be likely to say KITTY in the presence of a cat (so
there should be some correlation between the cat and the word KITTY).
The sounds [kIDi] correlate partially with the presence of cat
(so A predicts B). Doesn't that show that this is just an
indexical sign like those above? Unfortunately no - even if its true
that most early words for children are learned indexically (that is, by
pointing to what they refer to). In general, however, it is very rare
for the utterance of a word to correlate with the thing it refers to.
Sometimes such a correlation exists, of course, buta word in any
language is vastly more complex and sophisticated even for
language-learning infants. Notice that:

You and your baby will also freely use the word KITTY when
a cat is NOT around (so the correlation between KITTY and the
cat is a very weak). [If your dog knows the `word' TAKE-A-WALK, try
just discussing taking a walk in earshot of the dog and see what
happens! Dogs have no grasp of `Talking about taking-a-walk'.
That's because take-a-walk is only an indexical sign for
your dog, not a symbol as it is for you and your baby.]

Many words in every language describe objects that noone has ever
seen, like MONSTER, UNICORN, GHOST, DEVIL, etc. (so the possibility of
a ny correlation is ruled out completely)! What percent of the time
that you utter the word ROCKET or TRAIN, do you suppose there is a
physical rocket or train present? My guess is 0% for ROCKET and about
1-3% train. If there is no correlation or an extremely weak one, then
these words cannot be indices.

On the other hand, any word has strong associations with
other words that are `activated' whenever a word is heard
or read. Thus KITTY activates words like CAT, FUR, BABY, PURR, PUPPY,
PLAY, SAUCER, MILK, YARNBALL, CATFOOD, etc.
By `activate', I mean that you are more likely to
think of or utter these other words after hearing or saying KITTY.
(There are many kinds of experimental evidence for this, plus
intuition.) This suggests that KITTY may be somehow physically linked
to these other words in the brain. It suggests that KITTY gets some of
its meaning from the selective activation of just these particular
words (and their associated emotional content) when the word KITTY is
spoken.

These word-word relationships (sometimes called word-associates)
are critical for anchoring the meaning of a word without requiring any
correlation in space and time between the signal (the sound of the
word) and its meaning. Indices do not require any such set of
relationships to work as signs. In summary, symbols like most words in
a human language are (a) easily removable from their context,
and (b) are closely associated with large sets of other words.

Notice that humans easily learn words for things we have never
experienced. Children who grow up in the tropics learn to correctly
use words like SNOW and ICE without ever seeing snow or ice. It is not
a big problem for them because they have heard descriptions of them in
terms of words they do know, like COLD, WHITE, CLEAR, HARD,
SOFT, FLUFFY, WATER, MELT, FALLING, SLIPPERY, etc. From these
descriptions, they get a pretty good idea what snow and ice are like -
enough to read and produce the words appropriately.

This is the enormous power of human symbols: When you have learned a
basic vocabulary (based in part on indexical relationships), you can
use it to bootstrap to many other new concepts and words.
And given the possibility of cultural transmission from generation to
generation, human knowledge and understanding become cumulative and
have grown at a very rapid rate (relative to the creation and
transmission of innate knowledge).

IMPORTANT CLAIM:

Apparently no living nonhuman animals are able to
use word-like symbols.

There are, however, some (disputed) claims that a few individual
animals (mostly higher primates like monkeys, chimpanzees and gorillas)
have been trained by humans to use a small (< 50) inventory of
symbol-like units using hand signs or small physical tokens. If this
claim is true, it implies a huge divide between humans and nonhuman
animals. It means that no animal communication
systems can be understood as just `simple versions of human languages'. This claim is daring and provocative, but probably true. [Of
course, if one believes that humans are derived from nonhuman animals,
then somehow our ancestors must have passed through stages that were
intermediate between index-based communication systems (like dogs,
monkeys, bees, whales, etc) and modern-human symbolic language even
though we have very little direct evidence about how this evolution
took place.]

Nonword Symbols.

Words (especially nouns, verbs and adjectives) are the architype
for symbols. But the most common use of the term symbol in everyday,
nontechnical language is for signs that are not words: eg, a
flag or totem animal as the symbol of a country (bald eagle for USA,
bear for Russia, etc), a cross for Christianity, star of David for
Judaism, swastika for Nazism, a particular type font for a specific
product (eg, Coca-Cola, Indiana University, etc).

It seems that a similar set of associations to other words exist for
such symbols. Thus, the US FLAG (that is, the graphic pattern in red,
white and blue, not the English word FLAG) gets its meaning partly from
its association to words and concepts like: HOMELAND, WASHINGTON, BALD
EAGLE, PATRIOTISM, MOM, DAD, APPLE PIE, PRIDE, HEROISM, DEMOCRACY, `OH
SAY CAN YOU SEE...', `I PLEDGE ALLEGIANCE...', SACRIFICE, etc etc.

Mathematical and logical symbols also get their meaning from their
relation to other symbols. Thus pi is defined as the ratio of
the circumference of a circle to its diameter: pi = c/d.

So, nonword symbols are much like words but often lack a phonetic
form.

Conclusion.

The term sign is often used for all three of these: icons,
indices and symbols. All have a signal aspect,
some physical pattern (eg, a sound or visible shape) and a meaning
(some semantic content that is implied or `brought to mind') by the
signal. But they differ in that icons have a physical
resemblance between the signal and the meaning and an index
has a correlation in space and time with its meaning. But a symbol
is an arbitrary pattern (usually a sound pattern in a language)
that gets its meaning primarily from its mental association with
other symbolsand onlysecondarily from its
correlation with environmentally relevant properties.